The liquid phase diffusion bonding process bonds base materials by inserting therebetween an alloy in the form of a foil, a powder, a plated layer, etc. of a eutectic composition having a melting point lower than that of the base materials, pressing the base materials together, and heating the bonding portion to a temperature immediately above the solidus line of the inserted alloy (hereinafter referred to as an "insert metal") to cause melting and isothermal solidification; the process is considered as one type of solid phase bonding process. The isothermal solidification occurs because a specific element diffuses from a liquid phase to a material to be bonded (a base material), and thereby, the chemical composition of the liquid phase is shifted so that the solidus line of the shifted composition reaches the heating temperature.
The liquid phase diffusion bonding can be effected at relatively low pressing forces and is therefore not only used to avoid occurrence of a residual stress and a deformation due to bonding but is also applied to the bonding of high alloy steels and heat resistant steels which are difficult to bond and to the bonding of these steels with carbon steels.
On the other hand, carbon steels or steels for general use are conventionally bonded by other various bonding methods and the liquid phase diffusion bonding is rarely used.
The liquid phase diffusion bonding is frequently used to bond materials containing 0.50% in atomic percentage of Cr as an alloying element.
The Cr-containing material is characterized by having a dense coating film of a Cr oxide (mostly Cr.sub.2 O.sub.3) to provide high resistance to oxidation and corrosion.
Thus, it is naturally recognized that heating upon bonding also causes an oxide film to form on the bonding surfaces to impede leakage of a molten insert metal therethrough, and thereby, diffusion of atoms necessary for the bonding is significantly hampered to render it difficult to provide good bonded joints.
Thus, the conventional art such as Japanese Unexamined Patent Publications (Kokai) No. 53-81458, No. 62-34685, and No. 62-227595 necessarily use a vacuum or inert or reducing atmosphere during liquid phase diffusion bonding, which caused a significant increase in the bonding cost.
The present applicant has conducted various studies to achieve liquid phase diffusion bonding of stainless steels, high nickel-based alloys, and heat-resisting alloy steels and bonding between any of these materials and carbon steels in such a manner that the liquid phase diffusion bonding can be effected even in air, in which an oxide coating film is formed on the base material, to provide good bond joint quickly with a reduced cost and found that the use of an insert metal containing 0.1 to 20.0 at % V and an increased amount of Si enables liquid phase diffusion bonding in oxidizing atmospheres such as air.
Namely, the applicant found that, although V increases the melting point of the insert metal, an insert metal having good bondability can be obtained by adjusting the contents of other elements (Si alone in the present invention) and previously proposed a Ni-based, V- and Si-containing alloy foil for liquid phase diffusion bonding by enabling bonding in oxidizing atmospheres, mainly used for liquid phase diffusion bonding of stainless steels, high nickel-based alloys, and heat resistant alloy steels and bonding between any of these materials and carbon steels.
However, the above-proposed Ni-based alloy foil for liquid phase diffusion bonding is mainly used for bonding of stainless steels, high Ni-based alloys and heat resistant alloy steels and contains Ni as a main component.
Recently, there is increasing need to bond Fe-based materials represented by steel products such as steel pipes, steel reinforcements, steel plates or the like made of carbon steel in a narrow space in a reduced time to provide good joint strength with a low cost. The present inventors recognized that none of the conventional bonding methods can satisfy this need and studies feasibility of liquid phase diffusion bonding of Fe-base materials in air.
The inventors finally concluded that liquid phase diffusion bonding of Fe-based materials cannot be satisfactorily carried out with the use of the above-proposed Ni-based alloy foil for liquid phase diffusion bonding as an insert metal because the inclusion of a Ni-phase between base materials causes an inhomogeneous bond joint microstructure, requiring an elongated bonding time which affects the joint strength and toughness.